Optical scanning device

ABSTRACT

An optical scanning device ( 1 ) is for scanning two information layers ( 2, 2′ ) with two radiation beams ( 4 ) in two operating modes. It comprises a radiation source ( 7 ) for emitting the two radiation beams, an objective lens system ( 8 ) for converging the two beams on the positions of the two information layers, and a phase structure ( 24 ) having an optical axis ( 19 ) and including a central part (P 1 ) and at least one marginal part (P 2 ) for forming a non-periodic stepped profile in the radial direction. One (P 2 ) of said central and marginal parts (P 1 , P 2 ) includes at least two segments (AS 2,1 , AS 2,2 ) having two different step heights (h 2,1 , h 2,2 ), respectively, for introducing in said first operating mode two wavefront modifications  w 2,1,1? and  w 2,2,1?, respectively, and in said second operating mode two wavefront modifications  w 2,1,2? and  w 2,2,2?, respectively, where the difference ( w 2,1,2 +?w 2,2,2 )−(?w 2,1,1 +?w 2,2,1?) is asymmetric.

The present invention relates to an optical scanning device for scanninga first information layer by means of a first radiation beam in a firstoperating mode and a second information layer by means of a secondradiation beam in a second operating mode, the device comprising:

a radiation source for emitting said first and second radiation beamsconsecutively or simultaneously,

an objective lens system for converging said first and second radiationbeams on the positions of said first and second information layers,respectively, and

a phase structure arranged in the optical path of said first and secondradiation beams, the structure having an optical axis and including acentral part and at least one marginal part for forming a non-periodicstepped profile in the radial direction.

The present invention also relates to a phase structure for use in suchan optical scanning device.

“Scanning an information layer” refers to scanning by means of aradiation beam for reading information in the information layer(“reading mode”), writing information in the information layer (“writingmode”), and/or erasing information in the information layer (“erasemode”). “Information density” refers to the amount of stored informationper unit area of the information layer. It is determined by, inter alia,the size of the scanning spot formed by the scanning device on theinformation layer to be scanned. The information density may beincreased by decreasing the size of the scanning spot. Since the size ofthe spot depends, inter alia, on the wavelength λ and the numericalaperture NA of the radiation beam forming the spot. The size of thescanning spot can be decreased by increasing NA and/or by decreasing λ.

“First operating mode” refers to an operating mode of the opticalscanning device for scanning the first information layer by means of thefirst radiation beam. The first radiation beam has one or morepredetermined characteristics representative of the first mode, such as,inter alia, a wavelength λ₁, a polarization p₁, an angle of incidence i₁and/or a temperature T₁. “Second operating mode” refers to an operatingmode of the optical scanning device for scanning the second informationlayer by means of the second radiation beam. The second radiation beamhas one or more predetermined characteristics representative of thesecond mode, such as, inter alia, a wavelength λ₂, a polarization p₂, anangle of incidence i₂ and/or a temperature T₂, where the wavelengths λ₁and λ₂, the polarizations p₁ and p₂, the angles of incidence i₁ and i₂,and/or the temperatures T₁ and T₂ differ from each other. In otherwords, each operating mode may be characterized from another mode bymeans of at least one parameter having a first predetermined value forthat mode and a second, different value for the other mode(s).

A radiation beam propagating along an optical path has a wavefront Wwith a predetermined shape, given by the following equation:$\begin{matrix}{\frac{W}{\lambda} = \frac{\Phi}{2\pi}} & \left( {0a} \right)\end{matrix}$where “λ” and “Φ” are the wavelength and the phase of the radiationbeam, respectively.

“Wavefront aberration” refers to the following. A first optical elementwith an optical axis, e.g. an objective lens, for transforming an objectto an image may deteriorate the image by introducing the “wavefrontaberration” W_(abb). Wavefront aberrations have different typesexpressed in the form of the so-called Zernike polynomials withdifferent orders. Wavefront tilt or distortion is an example of awavefront aberration of the first order. Astigmatism and curvature offield and defocus are two examples of a wavefront aberration of thesecond order. Coma is an example of a wavefront aberration of the thirdorder. Spherical aberration is an example of a wavefront aberration ofthe fourth order. For more information on the mathematical functionsrepresenting the aforementioned wavefront aberrations, see, e.g. thebook by M. Born and E. Wolf entitled “Principles of Optics,” pp. 464-470(Pergamon Press 6^(th) Ed.) (ISBN 0-08-026482-4).

A “wavefront modification” refers to the following. A second opticalelement with an optical axis, e.g. a non-periodic phase structure, maybe arranged in the optical path of the radiation beam for introducing a“wavefront modification” ΔW in the radiation beam. The wavefrontmodification ΔW is a modification of the shape of the wavefront W. Likethe wavefront aberration, the wavefront modification may be symmetric orasymmetric, of a first, second, etc. order of a radius in thecross-section of the radiation beam if the mathematical functiondescribing the wavefront modification ΔW has a radial order of three,four, etc., respectively. The wavefront modification ΔW may also be“flat”; this means that the second optical element introduces in theradiation beam introduces a constant phase change so that, after takingmodulo 2π of the wavefront modification ΔW, the resulting wavefront isconstant. The term “flat” does not necessarily imply that the wavefrontW exhibits a zero phase change. Furthermore, it can be derived fromEquation (0a) that the wavefront modification ΔW may be expressed in theform of a phase change ΔΦ of the radiation beam, given by the followingequation: $\begin{matrix}{{\Delta\Phi} = {\frac{2\pi}{\lambda}\Delta\quad W}} & \left( {0b} \right)\end{matrix}$

“OPD” of a wavefront aberration W_(abb) or of a wavefront modificationΔW refers to the Optical Path Difference of the wavefront aberration ormodification. The root-mean-square value OPD_(rms) of the optical pathdifference OPD is given by the following equation: $\begin{matrix}{{OPD}_{rms} = \sqrt{\frac{\int{\int{{f\left( {r,\theta} \right)}^{2}r{\mathbb{d}r}}}}{\int{\int{r{\mathbb{d}r}{\mathbb{d}\theta}}}} - \left( \frac{\int{\int{{f\left( {r,\theta} \right)}r{\mathbb{d}r}{\mathbb{d}\theta}}}}{\int{\int{r{\mathbb{d}r}{\mathbb{d}\theta}}}} \right)^{2}}} & \left( {0c} \right)\end{matrix}$where “f” is the mathematical function which describes the wavefrontaberration W_(abb) or the wavefront modification ΔW and “r” and “θ” arethe polar coordinates of the polar coordinate system (r, θ) in a planenormal to the optical axis, with the origin of the system is the pointof intersection of that plane and the optical axis and extending overthe entrance pupil of the corresponding optical element.

Two values OPD_(rms,1) and OPD_(rms,2) are “substantially equal” to eachother where |OPD_(rms,1)−OPD_(rms,2)| is less than or equal to,preferably, 30 mλ, where the value 30 mλ has been chosen arbitrarily.Also, two values of phase changes ΔΦ_(a) and ΔΦ_(b) are “substantiallyequal” to each other where the respective values OPD_(rms,1) andOPD_(rms,2) are “substantially equal” to each other (the relationshipbetween ΔΦ and ΔW being given in Equation (0b)). Similarly, two valuesOPD_(rms,1) and OPD_(rms,2) (or two values of phase changes ΔΦ_(a) andΔΦ_(b)) are “substantially different” from each other where|OPD_(rms,1)−OPD_(rms,2)| is more than or equal to, preferably, 30 mλ.

A “symmetric” wavefront aberration or modification refers to a wavefrontaberration or modification that is rotationally symmetric over 2π withrespect an optical axis. For instance, defocus and spherical aberrationare symmetric aberrations.

An “asymmetric” wavefront aberration or modification refers to awavefront aberration or modification that is not a symmetric aberrationor modification as defined above. For instance, tilt, astigmatism andcoma are asymmetric aberrations. By way of illustration only, FIG. 1Ashows the shape of a comatic wavefront modification or aberrationΔW_(com) in a coordinate system (X₁, Y₁) and FIG. 1B show athe shape anastigmatic wavefront modification or aberration ΔW_(ast) in the samecoordinate system. As shown in FIG. 1A, the wavefront modification oraberration ΔW_(com) includes a first term “cos θ” for introducing coma.As shown in FIG. 1B, the wavefront modification or aberration ΔW_(ast)includes a second term “cos 2θ” for introducing astigmatism. It is notedin FIGS. 1A and 1B that the asymmetric modifications or aberrationsΔW_(com) and ΔW_(ast) are not rotationally symmetric with respect to thenormal to the XY-plane.

There is currently a need in the field of optical storage for providingoptical scanning devices for scanning optical records carriers by meansof radiation beams, where the devices are capable of compensatingwavefront aberrations arising in the optical path of the radiationbeams.

It has previously been proposed in, for example, the European Patentapplication filed on 22.07.2002 with the application number EP02077992.2, to provide an optical scanning device with a non-periodicphase structure (NPS) suitable for generating symmetric wavefrontmodifications. More specifically, the known scanning device is used forscanning a first information layer by means of a first radiation beam ina first operating mode and a second information layer by means of asecond radiation beam in a second operating mode. The known devicecomprises: a radiation source for emitting the two radiation beamsconsecutively or simultaneously, an objective lens system for convergingthe first and second radiation beams on the positions of the first andsecond information layers, respectively, and the NPS arranged in theoptical path of the first and second radiation beams. The known NPS hasan optical axis and includes a “central part”, i.e. a part which isarranged in an area centered on the optical axis, and a “marginal part”,i.e. a part which is arranged in an annular area around the centralpart. The central and marginal parts of the known NPS form anon-periodic stepped profile in the radial direction.

A drawback of the known optical scanning device provided with such anNPS is that it does not to introduce asymmetric wavefront modificationssuch as astigmatism, tilt, coma and clover. This is particularlydisadvantageous in the case where, e.g., the device is used for scanningtwo optical record carriers having two different formats, wherein theobjective lens includes birefringent material for correcting sphericalaberration arising in the radiation beam when scanning one of the twocarriers. During scanning of the other carrier, the birefringentobjective lens introduces astigmatism when the radiation beam does notpropagate along the optic axis of the birefringent material. Such anasymmetric aberration cannot be corrected by the known NPS, since thelatter can introduce a symmetric wavefront modification but not anasymmetric modification, such as an astigmatic wavefront modification,in the radiation beam.

Accordingly, it is an object to an optical scanning device operating inat least two modes, the device including an NPS for introducing anasymmetric wavefront modification in at least one of the first andsecond modes.

This object is reached by an optical scanning device as described in theopening paragraph wherein, according to the invention, one of saidcentral and marginal parts is divided into at least a first angularsegment and a second angular segment having a first step height and asecond, different step height, respectively, for introducing in saidfirst operating mode a first wavefront modification ΔW_(2,1,1) and asecond wavefront modification ΔW_(2,2,1), respectively, and in saidsecond operating mode a third wavefront modification ΔW_(2,1,1) and afourth wavefront modification ΔW_(2,2,2), respectively, where thedifference (ΔW_(2,1,2)+ΔW_(2,2,2))−(ΔW_(2,1,1)+ΔW_(2,2,1)) isasymmetric.

The first and second angular segments introduce the wavefrontmodifications ΔW_(2,1,2), ΔW_(2,2,2), ΔW_(2,1,1) and ΔW_(2,2,1) suchthat the difference between the resulting wavefront modificationΔW_(2,1,2)+ΔW_(2,2,2) in the second mode and the resulting wavefrontmodification ΔW_(2,1,1)+ΔW_(2,2,1) in the first mode is not rotationalsymmetric with respect to the optical axis of the NPS. By contrast, itis noted that the central and marginal parts of the known NPS introducein both the first mode and the second mode wavefront modifications thatare rotational symmetric with respect to the optical axis of that NPS,thereby introducing symmetric wavefront modifications, e.g. sphericalaberration and defocus.

Therefore, an advantage of the optical scanning device provided with thephase structure according to the invention is that it introduces anasymmetric wavefront modification in the radiation beam traversing theNPS during the second mode.

It is noted that U.S. Pat. No. 6,185,168B1 describes a phase structuredivided into a plurality of angular segments with respect to an opticalaxis. However, each segment has one stepped profile in the tangentialdirection so as to form a diffractive part introducing a wavefrontmodification that is rotationally symmetric with respect to the opticalaxis.

It is also noted that the known phase structure includes a diffractivepart while the phase structure according to the invention includes anon-periodic structure. Non-periodic structures and diffractive partsare different from each other in terms of structures and purposes. Thus,an NPS comprises a plurality of steps having different heights so thatthe NPS has a non-periodic profile in the radial direction. The latteris designed for forming a wavefront modification from a radiation beamincident to the NPS. By contrast, a diffractive part includes a patternof pattern elements having each one stepped profile: it has a periodicstepped profile in the radial or tangential direction. It is designedfor forming, from a radiation beam incident to the part, a diffractedradiation beam (i.e. a plurality of radiation beams having each adiffraction order “m”, i.e. the zeroth order (m=0), the +1^(st)-order(m=1), etc., the −1^(st)-order (m=−1), etc.) with different transmissionefficiencies for different diffraction orders.

Another object of the invention to provide a phase structure suitablefor use in an optical scanning device for scanning two informationlayers by means of two radiation beams in two operating modes, whereinthe phase structure introduces an asymmetric wavefront modification inat least one of the first and second operating modes.

This object is reached by a phase structure as described in the openingparagraph wherein, according to the invention, one of said central andmarginal parts includes at least a first segment and a second segmenthaving a first step height and a second, different step height,respectively, for introducing in said first operating mode a firstwavefront modification ΔW_(2,1,1) and a second wavefront modificationΔW_(2,2,1), respectively, and in said second operating mode a thirdwavefront modification ΔW_(2,1,2) and a fourth wavefront modificationΔW_(2,2,2), respectively, where the difference(ΔW_(2,1,2)+ΔW_(2,2,2))−(ΔW_(2,1,1)+ΔW_(2,2,1)) is asymmetric.

In accordance with another aspect of the invention, there is providedfor a lens for use in an optical scanning device for scanning a firstinformation layer by means of a first radiation beam in a firstoperating mode and a second information layer by means of a secondradiation beam in a second operating mode, the lens being provided witha phase structure according to the invention.

The objects, advantages and features of the invention will be apparentfrom the following, more detailed description of the invention, asillustrated in the accompanying drawings, in which:

FIGS. 1A and 1B show the shapes of a comatic wavefront modification andan astigmatic wavefront modification, respectively,

FIG. 2 is a schematic illustration of components of the optical scanningdevice according to the invention when operating in a first mode.

FIG. 3 shows an objective lens for use in the optical scanning deviceshown in FIG. 2, in the first mode and a second mode, FIG. 4 shows acurve representing an astigmatic aberration generated by the objectivelens shown in FIG. 3,

FIG. 5A shows a perspective view of the phase structure according to theinvention for compensating the astigmatic aberration shown in FIG. 4,

FIG. 5B shows a front view of the phase structure shown in FIG. 4,

FIG. 6 shows a surface representing the sum of the wavefront aberrationshown in FIG. 4 and of the wavefront modification introduced by thephase structure shown in FIGS. 5A and 5B,

FIG. 7 shows an improvement of the arrangement of the objective lens andphase structure shown in FIG. 3, and

FIGS. 8A and 8B shows two front views of two alternatives of the phasestructure shown in FIG. 5B.

An optical scanning device according to the invention is suitable forscanning optical record carriers having at least two different types orformats in at least a first operating mode, respectively. In thefollowing “S” is the total number of operating modes for the opticalscanning device; it is an integer equal to or higher than 2. Also “s”refers to the s-th operating mode of the device; it is an integercomprised between 1 and S.

FIG. 2 is a schematic illustration of components of the optical scanningdevice according to the invention, designated by the numeral reference1, when operating in the first mode. FIG. 3 shows an objective lens foruse in the optical scanning device 1.

As shown in FIG. 2, the optical scanning device 1 is capable of scanningin the first operating mode (s=1) a first information layer 2 of a firstoptical record carrier 3 having the first type by means of a firstradiation beam 4. It is also capable of scanning in the second operatingmode (s=2) a second information layer 2′ of a second optical recordcarrier 3′ by means of a second radiation beam 4′, as shown in FIG. 3.

By way of illustration, the optical record carrier 3 includes atransparent layer 5 on one side of which the information layer 2 isarranged. The side of the information layer facing away from thetransparent layer 5 is protected from environmental influences by aprotective layer 6. The transparent layer 5 acts as a substrate for theoptical record carrier 3 by providing mechanical support for theinformation layer 2. Alternatively, the transparent layer 5 may have thesole function of protecting the information layer 2, while themechanical support is provided by a layer on the other side of theinformation layer 2, for instance by the protective layer 6 or by anadditional information layer and transparent layer connected to theuppermost information layer. It is noted that the information layer hasa first information layer depth 27 that corresponds to, in thisembodiment as shown in FIG. 2, to the thickness of the transparent layer5. The information layer 2 is a surface of the carrier 3. That surfacecontains at least one track, i.e. a path to be followed by the spot of afocused radiation on which path optically-readable marks are arranged torepresent information. The marks may be, e.g., in the form of pits orareas with a reflection coefficient or a direction of magnetizationdifferent from the surroundings.

Similarly, the optical record carrier 3′ includes a second transparentlayer 5′ on one side of which the information layer 2′ is arranged witha second information layer depth 27′.

With reference to both FIGS. 2 and 3, the optical scanning device 1includes a radiation source 7, a collimator lens 18, a beam splitter 9,an objective lens system 8 having an optical axis 19, a phase structureor non-periodic structure (NPS) 24, and a detection system 10.Furthermore, the optical scanning device 1 includes a servocircuit 11, afocus actuator 12, a radial actuator 13, and an information processingunit 14 for error correction.

In the following “Z-axis” corresponds to the optical axis 19 of theobjective lens system 8. In the case where the optical record carriers 3and 3′ have the shape of a disc, the following is defined with respectto a given track: the “radial direction” is the direction of a referenceaxis, the X-axis, between the track and the center of the disc and the“tangential direction” is the direction of another axis, the Y-axis,that is tangential to the track and perpendicular to the X-axis. In thatcase (X, Y, Z) is an orthogonal base associated with the position of theinformation plane 2 and 2′.

The radiation source 7 consecutively or simultaneously supplies theradiation beams 4 and 4′. For example, the radiation source 7 maycomprise either a tunable semiconductor laser for consecutivelysupplying the radiation beams 4 and 4′ or two semiconductor lasers forsimultaneously supplying these radiation beams. Furthermore, theradiation beams 4 and 4′ have the first wavelength λ and the second,different wavelength λ₂, respectively.

In the present description that two wavelengths λ_(a) and λ_(b) aresubstantially different from each other where |λ_(a)−λ_(b)| is equal toor higher than, preferably, 10 nm and, more preferably, 20 nm, where thevalues 10 and 20 nm are a matter of a purely arbitrary choice.

It is noted that the first and second operating modes are characterized,by way of illustration only, by the wavelengths λ₁ and λ₂, respectively,where the wavelengths λ₁ and λ₂ substantially differ from each other.

The collimator lens 18 is arranged on the optical axis 19 as shown FIG.2. In the first mode it transforms the radiation beam 4 into a firstsubstantially collimated beam 20. In the second mode the collimator lens18 transforms the radiation beam 4′ into a second substantiallycollimated beam 20′ (not shown in FIG. 2).

The beam splitter 9 transmits in the first mode the collimated radiationbeam 20 toward the objective lens system 8. In the second mode ittransmits the collimated radiation beam 20′ toward the objective lenssystem 8 (not shown in FIG. 2). Preferably, the beam splitter 9 isformed with a plane parallel plate that is tilted with an angle α withrespect to the Z-axis and, more preferably, α=45°.

The objective lens system 8 transforms in the first mode the collimatedradiation beam 20 to a first focused radiation beam 15 having a firstnumerical aperture NA₁ so as to form a first scanning spot 16 in theposition of the information layer 2 (as shown in FIGS. 2 and 3). In thesecond mode the objective lens system 8 transforms the collimatedradiation beam 20′ to a second focused radiation beam 15′ having asecond numerical aperture NA₂ so as to form a second scanning spot 16′in the position of the information layer 2′ (as shown in FIG. 3).

As shown in FIGS. 2 and 3 and by way of illustration only, the objectivelens system 8 includes an objective lens 17 provided with the NPS 24(which will be described in further detail below).

It is noted in FIGS. 2 and 3 that the objective lens 17 is formed as ahybrid lens, i.e. a lens combining the NPS 24 and refractive elements,used in an infinite-conjugate mode. Such a hybrid lens can be formed byapplying a stepped profile on the entrance surface of the lens 17, forexample by a lithographic process using the photopolymerisation of,e.g., an UV curing lacquer, thereby advantageously resulting in the NPS24 to be easy to make. Alternatively, the objective lens 17 can be madeby diamond turning.

It is also noted in FIGS. 2 and 3 that the objective lens 17 is formedas a convex-convex lens; however, other lens element types such asplano-convex or convex-concave lenses can be used. Furthermore, theobjective lens 17 is a single lens. Alternatively, the objective lens 17may be a compound lens containing two or more lens element.

During scanning in the first mode, the record carrier 3 rotates on aspindle (not shown in FIGS. 2 and 3) and the information layer 2 is thenscanned through the transparent layer 5. The focused radiation beam 15reflects on the information layer 2, thereby forming a reflected beam 21which returns on the optical path of the forward converging beam 15. Theobjective lens system 8 transforms the reflected radiation beam 21 to areflected collimated radiation beam 22. The beam splitter 9 separatesthe forward radiation beam 20 from the reflected radiation beam 22 bytransmitting at least a part of the reflected radiation beam 22 towardsthe detection system 10. During scanning in the second mode, the recordcarrier 3′ rotates on a spindle (not shown in FIG. 3) and theinformation layer 2′ is then scanned through the transparent layer 5′.The focused radiation beam 15′ reflects on the information layer 2′,thereby forming a reflected beam 21′ which returns on the optical pathof the forward converging beam 15′. The objective lens system 8transforms the reflected radiation beam 21′ to a reflected collimatedradiation beam 22′. The beam splitter 9 separates the forward radiationbeam 20′ from the reflected radiation beam 22′ by transmitting at leasta part of the reflected radiation beam 22′ towards the detection system10.

The detection system 6 includes a convergent lens 25 and a quadrantdetector 23 for capturing, in the first mode, said part of the reflectedradiation beam 22 and, in the second mode, said part of the reflectedradiation beam 22′. The quadrant detector 23 converts, in the firstmode, the part of the reflected radiation beam 22 and, in the secondmode, the part of the reflected radiation beam 22, to one or moreelectrical signals. One of the signals is an information signalI_(data), the value of which represents, in the first mode, theinformation scanned on the information layer 2 and, in the second mode,the information scanned on the information layer 2′. The informationsignal I_(data) is processed by the information processing unit 14 forerror correction. Other signals from the detection system 10 are a focuserror signal I_(focus) and a radial tracking error signal I_(radial).The signal I_(focus) represents, in the first mode, the axial differencein height along the Z-axis between the scanning spot 16 and the positionof the information layer 2 and, in the second mode, the axial differencein height along the Z-axis between the scanning spot 16′ and theposition of the information layer 2′. Preferably, the signal I_(focus)is formed by the “astigmatic method” which is known from, inter alia,the book by G. Bouwhuis, J. Braat, A. Huijser et al, entitled“Principles of Optical Disc Systems,” pp. 75-80 (Adam Hilger 1985) (ISBN0-85274-785-3). The radial tracking error signal I_(radial) represents,in the first mode, the distance in the XY-plane of the information layer2 between the scanning spot 16 and the center of a track in theinformation layer 2 to be followed by the scanning spot 16 and, in thesecond mode, the distance in the XY-plane of the information layer 2′between the scanning spot 16′ and the center of a track in theinformation layer 2′ to be followed by the scanning spot 16′.Preferably, the signal I_(radial) is formed from the “radial push-pullmethod” which is known from, inter alia, the book by G. Bouwhuis, pp.70-73.

The servocircuit 11 is arranged for, in response to the signalsI_(focus) and I_(radial), providing servo control signals I_(control)for controlling the focus actuator 12 and the radial actuator 13,respectively. The focus actuator 12 controls the position of theobjective lens 17 along the Z-axis, thereby controlling, in the firstmode, the position of the scanning spot 16 such that it coincidessubstantially with the plane of the information layer 2 and, in thesecond mode, the position of the scanning spot 16′ such that itcoincides substantially with the plane of the information layer 2′. Theradial actuator 13 controls the position of the objective lens 17 alongthe X-axis, thereby controlling, in the fist mode, the radial positionof the scanning spot 16 such that it coincides substantially with thecenter line of the track to be followed in the information layer 2 and,in the second mode, the radial position of the scanning spot 16′ suchthat it coincides substantially with the center line of the track to befollowed in the information layer 2′.

The phase structure or NPS 24 according to the invention for use in theoptical scanning device 1 operating in the first and second modes is nowdescribed in further detail.

In the first embodiment shown in FIGS. 2 and 3, the NPS 24 is arrangedon the side of a first objective lens 17 facing the radiation source 7(referred to herein as the “entrance face”). (X_(o), Y_(o), Z_(o)) is anorthogonal base parallel to the base (X, Y, Z) and associated with theentrance surface of the objective lens 17, where the point of origin “O”is the center of the entrance pupil of the lens. As an alternative tothat embodiment, the NPS 24 may be arranged on the other surface of thelens 17 (referred to herein as the “exit face”). Also alternatively, theobjective lens 17 is, for example, a refractive objective lens elementprovided with a planar lens element forming the NPS 24. Alsoalternatively, the NPS 24 is provided on an optical element separatefrom the objective lens system 8, for example on a beam splitter or aquarter wavelength plate.

The NPS 24 includes a central part Pi and at least one marginal part P₂.In the present description a “central part” refers to part centered onan optical axis (the optical axis 19 in the present case) and having anouter boundary and a “marginal part” refers to a part located aroundsuch a central part and having an inner boundary and an outer boundary.In the following “NM” is the total number of the parts of the NPS 24; itis an integer equal to or higher than 2. “P_(m)” is the mth part of theNPS 24, where “m” is an integer comprised between 1 and M. It is notedthat “P_(m)” is central in the case where m=1 and annular in the casewhere m is equal to or higher than 2.

Furthermore, one of the parts P₁ and P₂, e.g. the part P₂, is dividedinto at least a first angular segment AS_(2,1) and a second segmentAS_(2,2). The first angular segment AS_(2,1) introduces a firstwavefront modification ΔW_(2,1,1) in the radiation beam 15 in the firstmode and a second wavefront modification ΔW_(2,1,1) in the radiationbeam 15′ in the second mode. The second angular segment AS_(2,2)introduces a third wavefront modification ΔW_(2,2,1) in the radiationbeam 15 in the first mode and a fourth wavefront modification ΔW_(2,2,2)in the radiation beam 15′ in the second mode. It is noted that the partP₂ introduces the resulting wavefront modification(ΔW_(2,1,1)+ΔW_(2,2,1)) in the radiation beam 15 in the first mode andthe resulting wavefront modification (ΔW_(2,1,2)+ΔW_(2,2,2)) in theradiation beam 15′ in the second mode. Additionally, the wavefrontmodifications ΔW_(2,1,1,) ΔW_(2,1,2), ΔW_(2,2,1) and ΔW_(2,2,2) are suchthat the difference (ΔW_(2,1,2)+ΔW_(2,2,2))−(ΔW_(2,1,1)+ΔW_(2,2,1))between the resulting wavefront modifications (ΔW_(2,1,2)+ΔW_(2,2,2))and (ΔW_(2,1,1)+ΔW_(2,2,1)) in the second and first modes is asymmetric,i.e. is not rotationally symmetric over 2π with respect to the opticalaxis 19.

In the following “J_(m)” is the total number of angular segments for thepart P_(m); it is an integer equal to or higher than 2. “AS_(m,j)” isthe jth angular segment of the part P_(m) where “j” is an integercomprised between 1 and J_(m). “h_(m,j)” is the step height of the jthangular segment of the part P_(m). “ΔW_(m,j,s)” is the wavefrontmodification introduced by the angular segment AS_(m,j) in the s-thoperating mode of the optical scanning device. “ΔΦ_(m,j,s)” is the phasechange associated with the wavefront modification ΔW_(m,j,s) accordingto Equation (0a). It is noted that the wavefront modification ΔW_(m,j,s)in the s-th mode is substantially constant for any point of thewavefront. It is also noted that the angular segment AS_(m,j) having thestep height h_(m,j) introduces the phase change ΔΦ_(m,j,s) in the s-thoperating mode as follows: $\begin{matrix}{{\Delta\Phi}_{m,j,s} = {\frac{2\pi}{\lambda_{s}}\left( {n - n_{o}} \right)h_{m,j}}} & \left( {2b} \right)\end{matrix}$where “λ_(s)” is the wavelength of the radiation beam traversing the NPS24 in the s-th mode and “n₀” is the refractive index of the adjacentmedium that is, in the following and by way of illustration only, air,i.e. n₀=1.

As an improvement of the NPS 24 described above, the step heightsh_(m,j) are chosen so that the NPS 24 introduces substantially flatwavefront modifications in one of the first and second mode, e.g. in thefirst mode. In the following the wavelength λ₁ of the radiation beam 15in the first mode refers to as the design wavelength λ_(ref).Consequently, the step heights h_(m,j) are chosen so that both thewavefront modifications ΔW_(1,1,1) and ΔW_(1,2,1) in the first mode aresubstantially flat (or the differences between phase changes ΔΦ_(m,j,1)in the first mode are substantially equal to different multiples of 2π,i.e. to zero modulo 2π) and the resulting wavefront modification(ΔW_(1,1,2)+ΔW_(1,2,2)) in the second mode is asymmetric.

In other words, the step heights h_(m,j) are chosen to be multiples of areference height h_(ref):h_(m,j)=q_(m,j)h_(ref)   (3a)where “q_(m,j)” is an integer and the reference height h_(ref) isdefined as follows: $\begin{matrix}{h_{ref} = {\frac{\lambda_{ref}}{n - n_{0}} = \frac{\lambda_{1}}{n - 1}}} & \left( {3b} \right)\end{matrix}$where “n” is the refractive index of the NPS 24. It is noted that theoptical path of the radiation beam traversing the NPS 24 depends on theangle of incidence of the beam entering the NPS 24 with respect to thenormal to the entrance pupil of the objective lens 17. Also, since thestep height h_(m,j) is a multiple of the reference height h_(ref), theoptical path of the radiation beam traversing the NPS 24 depend on therefractive index n according to Equation (3b) which in turn depends onthe temperature and the wavelength of the beam. In other words, theoptical path of the radiation beam traversing the NPS 24 in the firstmode differs from that in the second mode. Also, it is noted that thereference height h_(ref) is substantially constant, in the case wherethe NPS 24 is provided on a plane surface (e.g. on a plane parallelplate). Furthermore, in the case where the NPS 24 is provided on acurved surface (e.g. that of a lens), the NPS 24 may be adjusted overthe length of the step such that the phase changes ΔΦ_(m,j,s) aresubstantially equally to multiple of 2π, i.e. such that the curvature ofthe wavefront of the radiation beam entering the objective lens 17equals the curvature of the entrance surface of that lens.

A more specific embodiment of the improved NPS 24 is now described inthe case where the radiation beam 20 has a first polarization p₁ in thefirst mode, the radiation beam 20′ has a second, different polarizationp₂ in the second mode, and the objective lens 17 includes birefringentmaterial sensitive to the polarizations p₁ and p₂. In the following“r_(o)” is the pupil radius of the face of the objective lens 17, whichis provided with the NPS 24. Furthermore, the objective lens 17 isaligned such that the refractive index of the lens equals re in the casewhere the polarization p₁ or p₂ equals p_(e) and n_(o) in the case wherethe polarization p₁ or p₂ equals p_(o), where “n_(e)” and “n_(o)” arethe extraordinary and ordinary refractive indices of the birefringentmaterial. Also, the objective lens 17 introduces no aberration in thefirst mode and an astigmatic aberration W_(abb) in the second mode whenthe radiation beam does not propagate along the optic axis of thebirefringent material. FIG. 4 shows a curve 81 representing theastigmatic aberration W_(abb) in the coordinate system (X_(o), Y_(o)).In the following and by way of illustration only the value OPD_(rms) ofthe aberration W_(abb) equals 61 mλ.

In respect of the first mode the step heights h_(m,j) are chosen so thatthe wavefront modifications ΔW_(m,j,s) are substantially flat. Thus, thestep heights h_(m,j) are to equal different multiples q_(m,j) of thereference height h_(ref). By way of illustration only, in the case wherethe birefringent material is the quartz with n_(o)=1.54 and n_(e)=1.55,the wavelength λ₁ (i.e. the design wavelength) is equal to 405 nm andthe polarization p₁ equals p_(o), it is known from Equation (3b) thath_(ref)=0.751 μm.

In respect of the second mode and in the case where p₂=p_(e), n_(e)=1.55and λ₂=405 nm, the step heights h_(m,j) that equals q_(m,j)h_(ref)introduce the phase changes ΔΦ_(m,j,2) that differs from zero modulo 2π.Table I shows the values of the phase changes ΔΦ_(m,j,2) modulo 2π inthe second mode for different values of the integers q_(m,j). TABLE Iq_(m,j) ΔΦ_(m,j,2) (modulo 2π) 1 0.1160 2 0.2321 3 0.3481 4 0.4642 50.5802 6 0.6962

It is noted in Table I that the phase changes ΔΦ_(m,j,2) aresubstantially equal to a limited number of different values of phasechange modulo 2π. This limited number may be calculated based on thetheory of Continued Fractions, as known from, e.g., the European patentapplication filed on 05.04.2001 under the application number 01201255.5.

The integers q_(m,j) are chosen so that the NPS 24 compensates theastigmatic wavefront aberration W_(ast), i.e. so that the sum of theresulting wavefront modification$\sum\limits_{m = 1}^{M}{\sum\limits_{j = 1}^{J_{m}}{\Delta\quad W_{m,j,2}}}$and the astigmatic wavefront aberration W_(abb) substantially equalszero. Consequently, for reasons of symmetry, the NPS 24 includes threeparts, the central part P₁, the marginal P₂ and another marginal part P₃(M=3). The part P₁ includes one angular segment AS_(1,1) (J₁=1), thepart P₂ includes four angular segments AS_(2,1) AS_(2,2) AS_(2,4) (J₂=4)and the part P₃ includes four angular segments AS_(3,1) AS_(3,2,)AS_(3,3) and AS_(3,4) (J₃=4)

Each of the angular segments AS_(m,j) is defined by a minimum radiusr_(min)(AS_(m,j)), a maximum radius r_(max)(AS_(m,j)), a minimum angleθ_(min)(AS_(m,j)) and a maximum angle θ_(max)(AS_(m,j)). It is noted inrespect of that improved embodiment of the NPS 24 that the angularsegments AS_(m,j) are rotationally symmetric with respect to theZ_(o)-axis (i.e. the optical axis of the objective lens 17) over therange of angles comprised between the minimum and maximum anglesθ_(min)(AS_(m,j)) and θ_(max)(AS_(m,j)). Table II shows the “optimizedzones” of the angular segments AS_(m,j), i.e. the radiir_(min)(AS_(m,j)) and r_(max)(AS_(m,j)) and the angles θ_(min)(AS_(m,j))and θ_(max)(AS_(m,j)). Also Table II refers to the case where p₁=p_(o)and the phase changes ΔΦ_(m,j,2) given in Table I and the wavefrontaberration W_(abb) (see FIG. 4) according to the method known, e.g.,from the article by B. H. W. Hendriks, J. E. de Vries and H. P. Urbach,“Application of non-periodic phase structures in optical systems”, Appl.Opt. 40 (2001) pp. 6548-6560, which describes how to make a objectivelens suitable for scanning DVD-format discs and CD-format discs with theaid of an NPS. TABLE II Parts Angular segments P_(m) AS_(m,j)r_(min)/r_(o) r_(max)/r_(o) θ_(min) [°] θ_(max) [°] P₁ AS_(1,1) 0 0.333−45 +315 P₂ AS_(2,1) 0.333 0.733 −45 +45 AS_(2,2) 0.333 0.733 +45 +135AS_(2,3) 0.333 0.733 +135 +225 AS_(2,4) 0.333 0.733 +225 +315 P₃AS_(3,1) 0.733 1 −45 +45 AS_(3,2) 0.733 1 +45 +135 AS_(3,3) 0.733 1 +135+225 AS_(3,4) 0.733 1 +225 +315

Table III shows the step heights h_(m,j) for the angular segmentsAS_(m,j) as defined in Table II. TABLE III Parts P_(m) Angular segmentsAS_(m,j) Step heights h_(m,j) [μm] P₁ AS_(1,1) 0 P₂ AS_(2,1) −1.50AS_(2,2) 1.50 AS_(2,3) −1.50 AS_(2,4) 1.50 P₃ AS_(3,1) −3.75 AS_(3,2)3.75 AS_(3,3) −3.75 AS_(3,4) 3.75

It is noted in Table III that some step heights have negative values,e.g. in respect of the angular segment AS_(2,1). In the presentdescription a negative value of a step height indicates a depression,instead of a rise, in the body having the non-periodic stepped profile,here the objective lens 17.

For further detail, see the European patent application EP 1,179,212-Athat describes a negative step height of a plate.

FIG. 5A shows a perspective view of the NPS 24 designed according toTable III. FIG. 5B shows a front view of that NPS viewed from the sideof the entrance pupil of the objective lens 17. It is noted in FIG. 5Bthat the angular segments are arranged according to aquadrant-arrangement where the dividing lines between the angularsegments AS_(2,j) are aligned with those between the angular segmentsAS_(3,j). It is also noted that the stepped profile of the NPS 24 isdesigned such that the relative step heights h_(m,j+1)-h_(m,j) orh_(m+1,j)-h_(m,j) between adjacent angular segments S_(m,j+1) andS_(m,j) or S_(m+1,j) and S_(m,j), respectively, include a relative stepheight having an optical path substantially equal to aλ₁, wherein “a” isan integer and a>1 and “λ₁” is the design wavelength. In other words,such a relative step height is higher than the reference height h_(ref).

FIG. 6 shows a surface 82 representing the sum${\sum\limits_{m = 1}^{3}{\sum\limits_{j = 1}^{J_{m}}{\Delta\quad W_{m,j,2}}}} + W_{abb}$of the wavefront aberration W_(abb) shown in FIG. 4 and the wavefrontmodification$\sum\limits_{m = 1}^{3}{\sum\limits_{j = 1}^{J_{m}}{\Delta\quad W_{m,j,2}}}$introduced by the NPS 24 shown in FIGS. 5A and 5B.

Table IV shows the values${{OPD}_{rms}\left\lbrack W_{abb} \right\rbrack}\quad{and}\quad{{OPD}_{rms}\left\lbrack {{\sum\limits_{m = 1}^{M}{\sum\limits_{j = 1}^{J_{m}}{\Delta\quad W_{m,j,2}}}} + W_{abb}} \right\rbrack}$

without and with the NPS 24 shown in FIG. 5A for compensating thewavefront aberration W_(abb) shown in FIG. 4, respectively. These valueshave been calculated from ray-tracing simulations. TABLE IVOPD_(rms)[W_(abb)]${OPD}_{rms}\left\lbrack {{\sum\limits_{m = 1}^{M}{\sum\limits_{j = 1}^{J_{M}}{\Delta W}_{m,j,2}}} + W_{abb}} \right\rbrack$61 mμ 30 mμ

It is noted that the NPS 24 shown in FIGS. 5A and 5B compensatesapproximately 50% of the astigmatic aberration W_(abb) introduced by thebirefringent objective lens 17. It is also noted in Table IV that thevalue${OPD}_{rms}\left\lbrack {{\sum\limits_{m = 1}^{M}{\sum\limits_{j = 1}^{J_{M}}{\Delta\quad W_{m,j,2}}}} + W_{abb}} \right\rbrack$is below the diffraction limit, i.e. less than 70 mλ, for the NPS 24according to Table IV, thereby allowing any format of optical recordcarriers to be scanned with a significant improvement in terms ofOPD_(rms) with respect to the case where an optical scanning device isnot provided with such a NPS.

It is also noted that a birefringent lens such as the lens 17 providedwith the NPS 24 as described above forms a lens system for forminginfinite conjugate, e.g. for dual layers. One of those two informationlayers is scanned in the first mode by means of a radiation beam havinga predetermined polarization and the other information layer is scannedin the second mode by means of another radiation beam having a differentpolarization. Thus, while the birefringent lens introduces astigmatismin the second mode and no astigmatism in the first mode, the NPSaccordingly compensates such astigmatism in the second mode and isoptically inactive in the first mode.

Whilst in the above described embodiment an optical scanning devicecompatible with a birefringent objective lens is described, it is to beappreciated that the scanning device according to the invention can bealternatively used for any other types of optical record carriers to bescanned. It is noted that the use of a birefringent material is notessential for the invention.

An alternative of the phase structure described above is designed forintroduced any asymmetric wavefront modification, e.g. a comaticwavefront modification as shown in FIG. 1A. For more information on themathematical functions representing such wavefront modifications, see,e.g. the book by M. Born and E. Wolf entitled “Principles of Optics,”pp. 464-470 (Pergamon Press 6^(th) Ed.) (ISBN 0-08-026482-4).

Also alternatively, the phase structure may be provided with numbers ofparts and angular segments other than those described above. It is notedthat the higher the number M and/or the numbers J_(m) the better thecompensation of the aberration W_(abb) introduced by the objective lens.However this would result in increasing the complexity of the NPSespecially in terms of manufacturing. Therefore a tradeoff betweencompensation and manufacturing need be found.

Also alternatively, the angular segments are provided in an arrangementother than the quadrant-arrangement shown in FIG. 5B. FIGS. 8A and 8Bshows two front views of two alternatives of the phase structure shownin FIG. 5B. As shown in FIG. 8A the NPS 24′ has two parts P′₁ andP′_(2,) wherein the part P′₁ is divided into two angular segmentsAS′_(1,1) and AS′_(1,2) and the part P′₂ is divided into two angularsegments AS′_(2,1) and AS′_(2,2). Similarly, the NPS 24″ shown in FIG.8B has two parts P″₁ and P″₂, and four angular segments AS″_(1,1,)AS″_(1,2), AS″_(2,1) and AS″_(2,2). It is noted that the dividing linebetween the angular segments AS′_(1,1) and AS′_(1,2) is aligned with thedividing line between the angular segments AS′_(2,1) and AS′_(2,2) whilethe dividing line between the angular segments AS″_(1,1) and AS″_(1,2)forms a non-zero angle α with the dividing line between the angularsegments AS″_(2,1) and AS″_(2,2). In other words, these differentpossible arrangements of the parts of the NPS allow to introduce anyasymmetric wavefront modification in the radiation beam traversing theNPS.

Another alternative to the phase structure arranged on the entrance faceof the objective lens may be of any shape like a plate. Furthermore, asan improvement of such a plate provided with the NPS according to theinvention, a cover layer is arranged on the angular segments such thatthe phase structure forms a plane plate. FIG. 7 shows such animprovement where a plate 89 including the NPS 24 is provided with acover layer 90. By way of illustration only, the cover layer 90 is madeof an isotropic material having a refractive index that substantiallyequals the refractive index n_(o) of the birefringent material of theplate 89. In the present description, two refractive indices n_(a) andn_(b) are substantially equal where|n_(a)−n_(b)| is equal to or lessthan, preferably, 0.01 and, more preferably, 0.005, where the values0.01 and 0.005 are a matter of arbitrary choice. In the first mode thepolarization of the radiation beam traversing the improved NPS 24 issuch that the refractive index of the NPS 24 equals n_(o). In that modethe improved NPS 24 has no optical effect on the radiation beamtraversing the NPS. In the second mode the polarization of the radiationbeam traversing the NPS 24 is such that the refractive index of the NPS24 equals n_(e). In that mode there is a mismatch between the refractiveindices of the birefringent plate 89 and the isotropic cover layer 90.For generating the phase change ΔΦ_(m,j,2) the step height h_(m,j) mustequal h_(m,j)=h_(ref)ΔΦ_(m,j,2)/2π, where h_(ref)=λ_(ref)/(n_(e)−n_(o)).Thus, by way of illustration only and in the case where n_(o)=11.5,n_(e)=1.6 and λ_(ref)=405 nm, it is found that h_(ref)=4.05 μm. Byproper design of the various heights h_(m,j) the NPS 24 generates anastigmatic resulting wavefront modification$\sum\limits_{m = 1}^{M}{\sum\limits_{j = 1}^{J_{m}}{\Delta\quad{W_{m,j,2}.}}}$

Table V shows the step heights h_(m,j) for the angular segments AS_(m,j)as defined in Table II. TABLE V Parts P_(m) Angular segments AS_(m,j)Step heights h_(m,j) [μm] P₁ AS_(1,1) 0 P₂ AS_(2,1) −0.1218 AS_(2,2)0.1218 AS_(2,3) −0.1218 AS_(2,4) 0.1218 P₃ AS_(3,1) −0.3039 AS_(3,2)0.3039 AS_(3,3) −0.3039 AS_(3,4) 0.3039

Table VI shows the values${{OPD}_{rms}\left\lbrack W_{abb} \right\rbrack}\quad{and}\quad{{OPD}_{rms}\left\lbrack {{\sum\limits_{m = 1}^{M}{\sum\limits_{j = 1}^{J_{M}}{\Delta\quad W_{m,j,2}}}} + W_{abb}} \right\rbrack}$

without and with the NPS 24 as given in Table V for compensating thewavefront aberration W_(abb) shown in FIG. 4, respectively. These valueshave been calculated from ray-tracing simulations. TABLE VI 4OPD_(rms)[W_(abb)]${OPD}_{rms}\left\lbrack {{\sum\limits_{m = 1}^{M}{\sum\limits_{j = 1}^{J_{M}}{\Delta W}_{m,j,2}}} + W_{abb}} \right\rbrack$61 mλ 28 mλ

It is noted that the value${OPD}_{rms}\left\lbrack {{\sum\limits_{m = 1}^{M}{\sum\limits_{j = 1}^{J_{M}}{\Delta\quad W_{m,j,2}}}} + W_{abb}} \right\rbrack$corresponding to the compensation with the NPS 24 as given in Table V islower than the value${OPD}_{rms}\left\lbrack {{\sum\limits_{m = 1}^{M}{\sum\limits_{j = 1}^{J_{M}}{\Delta\quad W_{m,j,2}}}} + W_{abb}} \right\rbrack$given in Table IV (i.e. 30 mλ). In other words, the NPS 24 provided withthe cover layer 90 improves the compensation of the wavefront aberrationW_(abb) with respect to the NPS 24 without the cover layer 90.

Also alternatively, a plurality of phase structures according to theinvention may be provided with different optical elements or in onesingle body, wherein the phase structures have different materials.

Alternatives to the optical scanning device described above operate atmodes different from the first and second mode described above, forinstance in two modes with two different wavelengths (as described inthe international application WO 01/48746 published on Jul. 5, 2001), intwo modes with two different temperatures (as described in the Europeanpatent application EP 1.179.212 published on Feb. 13, 2002), two modeswith two different angles of incidence (as described in said article byB. H. W. Hendriks et al., or in three modes with three differentwavelengths (as described in the European patent application filed underthe application number 02077992.2 on 22.07.2002).

As another alternative of the optical scanning device described above,at least one of the polarizations of the radiation beams traversing theNPS is switched between a first state and a second state such that theNPS introduces a flat wavefront modification when that polarization isin the first state and an asymmetric wavefront modification when thatpolarization is in the second state. It is noted that the switching ofeach of the polarizations is known, e.g., from the European Patentapplication filed on 07.12.2001 with the application number EP01204786.6.

Alternatively, at least one of the polarizations is switched between afirst state and a second state such that the NPS introduces a firstasymmetric wavefront modification when that polarization is in the firststate and a second, different wavefront modification when thatpolarization is in the second state.

It is noted that the phase structure described above in relation to anoptical scanning device may be used for other optical applications, e.g.in microscopy and photography, operating in a first mode and in a secondmode.

1. An optical scanning device for scanning a first information layer bymeans of a first radiation beam in a first operating mode and a secondinformation layer by means of a second radiation beam in a secondoperating mode, the device comprising: a radiation source for emittingsaid first and second radiation beams consecutively or simultaneously,an objective lens system for converging said first and second radiationbeams on the positions of said first and second information layers,respectively, and a phase structure arranged in the optical path of saidfirst and second radiation beams, the structure having an optical axisand including a central part (PI) and at least one marginal part (P₂)for forming a non-periodic stepped profile in the radial direction,characterized in that one (P₂) of said central and marginal parts (P₁,P₂) is divided into at least a first angular segment (AS_(2,1)) and asecond angular segment (AS_(2,2)) having a first step height (h_(2,1))and a second, different step height (h_(2,2)), respectively, forintroducing in said first operating mode a first wavefront modificationΔW_(2,1,1) and a second wavefront modification ΔW_(2,2,1,) respectively,and in said second operating mode a third wavefront modificationΔW_(2,1,1) and a fourth wavefront modification ΔW_(2,2,2), respectively,where the difference (ΔW_(2,1,2)+ΔW_(2,2,2))−(ΔW_(2,1,1)+ΔW_(2,2,1)) isasymmetric (19).
 2. An optical scanning device according to claim 1,wherein (ΔW_(2,1,2)+ΔW_(2,2,2))−(ΔW_(2,1,1)+ΔW_(2,2,1)) is of the typeof astigmatism, tilt, coma or clover.
 3. An optical scanning deviceaccording to claim 1, wherein either the resulting wavefrontmodification ΔW_(2,1,1)+ΔW_(2,2,1) in the first mode or the resultingwavefront modification ΔW_(2,1,2)+ΔW_(2,2,2) in the second mode issubstantially flat.
 4. An optical scanning device according to claim 1,wherein said phase structure includes birefringent material sensitive toa first polarization (p₁) of said first radiation beam in said firstmode and to a second, different polarization (p₂) of said secondradiation beam in said second mode.
 5. An optical scanning deviceaccording to claim 1, further including a cover layer arranged such thatsaid phase structure forms a plate.
 6. An optical scanning deviceaccording to claim 1, wherein said heights are designed such that therelative step heights (h_(m,j+1)−h_(m,j); h_(m+1,j)−h_(m,j)) betweenadjacent steps (AS_(m,j+1), AS_(m,j); AS_(m+1,j), AS_(m,j)) include arelative step height having an optical path substantially equal to aλ₁,wherein “a” is an integer and a>1 and “λ₁ 38 is the wavelength of saidfirst radiation beam.
 7. An optical scanning device according to claim1, wherein said phase structure is generally circular and said steps aregenerally annular.
 8. An optical scanning device according to claim 1,wherein said phase structure is formed on a face of a lens of saidobjective lens system.
 9. An optical scanning device according to claim1, wherein said phase structure is formed on an optical plate providedbetween said radiation source and said objective lens system.
 10. Anoptical scanning device according to claim 9, wherein said optical platecomprises a quarter wavelength plate or a beam splitter.
 11. A phasestructure operating in a first mode and in a second mode, the structurehaving an optical axis and including a central part (P₁) and at leastone marginal part (P₂) for forming a non-periodic stepped profile in theradial direction, characterized in that one (P₂) of said central andmarginal parts (P₁, P₂) includes at least a first segment (AS_(2,1)) anda second segment (AS_(2,2)) having a first step height (h_(2,1)) and asecond, different step height (h_(2,2)), respectively, for introducingin said first operating mode a first wavefront modification ΔW_(2,1,1)and a second wavefront modification ΔW_(2,2,1), respectively, and insaid second operating mode a third wavefront modification ΔW_(2,1,2) anda fourth wavefront modification ΔW_(2,2,2), respectively, where thedifference (ΔW_(2,1,2)+ΔW_(2,2,2))−(ΔW_(2,1,1)+ΔW_(2,2,1)) isasymmetric.
 12. A lens for use in an optical scanning device forscanning a first information layer by means of a first radiation beam ina first operating mode and a second information layer by means of asecond radiation beam in a second operating mode, the lens beingprovided with a phase structure according to claim 11.